Rotary offset printing machines have been used for a number of years. The basic mechanisms, principles, and steps of operation for modern rotary printers include chemically forming an image on a thin metal image plate. The thin image plate therearound is attached around the circumference of a plate cylinder. Ink and a water solution are applied by inking unit rollers and dampening unit rollers to the respective chemically treated areas that are to form an image on the image plate as it rotates with the plate cylinder. The plate cylinder rolls the image plate against a blanket cylinder offsetting a reverse image around the circumference of the blanket cylinder. A sheet of print paper or other material to be printed is fed into the press and gripped by an impression cylinder. The impression cylinder pulls the paper into rolling contact between the bluet cylinder and the impression cylinder. Under rolling pressure between the two cylinders, the image is imprinted from the blanket cylinder onto the paper. The imprinted image is the reverse of that on the bluet cylinder so that it is appears as originally formed on the image plate. After the paper is imprinted, it is removed from the impression cylinder gripper and transferred to a collection tray, if printing is finished. If additional colors or images are to be applied, the sheet may be transferred to one or more other impression cylinders which grip the print paper from a transfer gripper and roll the paper against a subsequent blanket cylinder for additional printing. Alternatively, in some presses, a large impression cylinder carries the sheet past one or more additional blazer cylinders which roll the next color onto the sheet.
In multi-color offset printers, each color is applied as a series of minute dots or patterns. It is extremely important to precisely locate or register the paper as it is gripped by the impression cylinders, or each of them, so that each subsequent matrix of colored dots can be properly located and coordinated with respect to other color dots to form the desired image. Quality printing requires precise location of the dots within thousandths of an inch of each other. An error in alignment of a few thousandths of an inch can produce a blurred image or an image with improperly mixed and overlapping color dots.
In the past, multiple color offset rotary printers accomplished this precise registration through careful attention to the transfer of the paper using precisely manufactured fixed diameter transtar cylinders. Usually, the transfer cylinders are large enough to carry two sheets of print paper spaced end-to-end around the circumference of the cylinder. The rotation of the transfer cylinders had to be carefully timed with respect to the rotation of the impression cylinders so that the paper when picked up by the transfer cylinders from one of the impression cylinders was carried around the transfer cylinder at precisely the correct speed and distance so that it was gripped by a subsequent impression cylinder precisely in the correct location for registration. The grippers for each impression cylinder had to be adjusted until the dots were printed precisely at the desired location.
The use of transfer cylinders has been important because of the extreme criticality of precise registration. The cylinders, once formed, have a fixed diameter and can be rotated through gears at a fixed speed. Repeatable transfers are thus made possible. However, this structure is complex and expensive. Further, it introduces associate problems. For example, smearing can result because the printed surface of the paper being transferred is directed inward on each transfer cylinder. Thus, the printed surface of the sheet faces outward toward the blanket cylinder when it is gripped by the next impression cylinder. Special coatings, special non-stick screens, and even complex systems for air cushioning the paper as it is carried around the transfer cylinder have been employed in order to minimize this smearing problem.
The cost of manufacturing multiple color offset printers has been very high because of the complexity of multiple transfer gripping mechanisms, large precision-built transfer cylinders, and non-smear mechanisms. Further, because of the need to properly adjust registration of the paper as it is received by each impression cylinder, transferred to each transfer cylinder and then received by each subsequent impression cylinder, the time and expense to set up any given multiple color offset printing job has been substantial. It is not uncommon for an operator to spend a considerable amount of time setting up a job and to use over five hundred (500) trial printing sheets before proper registration is obtained for all of the color impression cylinders. As a result, multiple color offset rotary printing has not been economically feasible for most small printing jobs requiring less than several thousand copies.
Traditional rotary offset printing presses employ inking units, including sets of rollers, which rollers roll against each other and against the printing press plate cylinders. The rollers of the inking unit carry ink from an ink source for distributing and applying appropriate quantities of a desired colored ink onto the plate cylinder. The plate cylinder holds the printing plate which attracts the ink to an image to be printed. The ink image is transferred to the blanket cylinder for imprinting it onto individual sheets of paper which are held against an impression cylinder. There is often limited access area to the plate cylinder, the blanket cylinder and impression cylinder of the rotary printing press. It has been found to be desirable to allow the inking unit to be movable for purposes of allowing easier access to the main cylinders of the printing press.
Various devices have been developed with varying degrees of success for accomplishing the task of allowing access to the main cylinder of printing presses. One such device was disclosed in U.S. Pat. No. 4,896,599, issued to the present applicant, James J. Keller, titled Swing-Away Colorhead for Offset Duplicator. Such a device provided many advantages over fixed inking unit systems. For example, that device would allow access to the plate cylinder, or to the other internal mechanisms of the press. Still, there continues to be a need for a mechanism by which the entire inking unit could be conveniently moved for access withore removing the inking unit entirely, and also a mechanism which would allow the inking unit to be removed and replaced with another inking unit (e.g. an inking unit set up with a different color). Also, there was a need for providing a convenient mechanism and system by which an inking unit having been operated with one color of ink could be cleaned for use with another color of ink.